1. Field of the Invention
The invention relates to a method of foaming a viscous fluid. The foamed fluid is utilized in the stimulation of subterranean formations. The method also can be utilized in gravel packing operations, well bore cleanouts and the like.
2. Description of the Prior Art
The treatment of subterranean formations penetrated by a well bore to increase the productivity of the well is known in the art. Various methods are known to achieve increased productivity from a subterranean formation. Such methods include well stimulation, both to initially stimulate a well and to improve production from producing wells, formation stabilization, well bore cleanouts of producing wells and the like. With regard to well stimulation, emphasis has been placed on hydraulically fracturing formations with various liquids, which may contain propping agents suspended therein. When hydraulic pressure is applied to a subterranean formation by injection of a fluid into a well bore penetrating the formation, the pressure of the fluid creates tensile stresses in the rock of the formation. The stresses ultimately cause splitting, parting or fracturing of the rock. The initially formed fracture or fractures can be extended in the formation by continued injection of fluid under sufficient pressure into the formation. The fractures can be maintained in a partially open position by the placement of propping agents into the fractures or by acid etching techniques. When a propping agent is employed, it is introduced into the fracture in admixture with the treating fluid. When the pressure is released, the fractures in the formation close upon the etched faces of the fracture or the propping agent to create productive channels in the subterranean formation. The quantity of propping agent in the fluid is of significance because it effects the open width and height of the propped fracture.
Hydraulic fracturing with foamed fluids is a recent development. This process generally is known as "foam fracturing" and is generally described in U.S. Pat. No. 3,937,283. Briefly, the foam fracturing process involves generation of a foam of a desired quality. The quality is defined as the ratio of the volume of gas to the volume of foamed gas and liquids present under the conditions existing in the subterranean formation expressed as a percentage. Generally, it has been found desirable to utilize foams having a quality in the range of from about 50 to 99 percent. Various gases and liquids may be used to create the foam, but the usual foams are made from nitrogen or carbon dioxide and water in the presence of a suitable foaming agent. The foam is pumped into the formation at a temperature and pressure sufficient to cause a fracture in the subterranean formation.
Fracturing caused by the foam fracturing process is very effective and has many advantages over the known prior art methods. For example, foam fracturing can be carried out without significant fluid loss in comparison to treatments utilizing unfoamed liquids. Additionally, the foam comes out of the well easily when pressure is removed from the well head, because of the expansion of the foam when the pressure is released.
Foams produced in accordance with the prior art have one undesirable property, the maximum proppant concentration which can be obtained is quite low. Conventionally prepared foams generally can transport one to two pounds of proppant per gallon of foam. The proppant concentration can be increased by admixing gelling agents with the fluid which is to be foamed. However, the presence of a viscosifying agent in the fluid further increases the quantity of mechanical energy required to foam the fluid. It would be desirable to provide a means by which a foam can be produced which is capable of transporting a significant quantity of proppant into a subterranean formation with a minimum expenditure of mechanical energy.